A local area network (LAN) is an electronic system which includes several devices called stations, modules or nodes which interact with each other to perform specific tasks. The modules on the network are interconnected by a bus which is often implemented by a very sophisticated system of cables. The modules communicate over the bus in accordance with standardized protocols.
One type of network communication protocol is known as token passing. During normal operation on a token passing LAN, a package of data called a token is passed or transmitted along the bus (in this case referred to as the token bus) from one module to the next in succession. In a ring type token passing LAN, one can visualize the token being passed from module to module repeatedly around the ring. The token is used to control access to the bus. Each module has possession of the token for a specific predetermined period of time. During that period, the module in possession has exclusive use of the token bus to communicate with other modules, peripheral devices, etc.
Local area networks are often used to control operations in an industrial manufacturing or process plant. Very large networks are formed which may incorporate multiple user workstations, control and applications processors as well as all of the electrical, mechanical, hydraulic, pneumatic or other type of equipment used to perform the various tasks required by the specific process being controlled. It will be appreciated that in such systems reliability of the network is a high priority requirement.
Traditionally, failures of cables and modules in such systems have been difficult to detect and even more difficult to correct expeditiously. Such failures have resulted in lengthy and costly process shutdowns while diagnostic and remedial tasks were performed.